Illumination systems have a variety of applications, including projection displays, backlights for liquid crystal displays (LCDs) and others. Projection systems usually include a source of light, illumination optics, an image-forming device, projection optics and a projection screen. The illumination optics collect light from a light source and direct it to one or more image-forming devices in a predetermined manner. The image-forming device(s), controlled by an electronically conditioned and processed digital video signal, produces an image corresponding to the video signal. Projection optics then magnify the image and project it onto the projection screen. White light sources, such as arc lamps, in conjunction with color wheels have been and still are predominantly used as light sources for projection display systems. However, recently, light emitting diodes (LEDs) were introduced as an alternative. Some advantages of LED light sources include longer lifetime, higher efficiency and superior thermal characteristics.
One example of an image-forming device frequently used in digital light processing systems is a digital micro-mirror device (DMD). The main feature of a DMD is a rectangular array of rotatable micro-mirrors. The tilt of each mirror is independently controlled by the data loaded into the memory cell associated with each mirror, to steer reflected light and spatially map a pixel of video data to a pixel on a projection screen. Light reflected by a mirror in an ON state passes through the projection optics and is projected onto the screen to create a bright field. On the other hand, light reflected by a mirror in an OFF state misses the projection optics, which results in a dark field. A color image also may be produced using a DMD, e.g., utilizing color sequencing, or, alternatively, using three DMDs, one for each primary color.
Other examples of image-forming devices include liquid crystal panels, such as a liquid crystal on silicon device (LCOS), which are typically rectangular. In liquid crystal panels, the alignment of the liquid crystal material is controlled incrementally (pixel-to-pixel) according to the data corresponding to a video signal. Depending on the alignment of the liquid crystal material, polarization of the incident light may be altered by the liquid crystal structure. Thus, with appropriate use of polarizers or polarizing beam splitters, dark and light regions may be created, which correspond to the input video data. Color images have been formed using liquid crystal panels in the manner similar to the DMDs.
Performance of optical systems, such as illumination optics of a projection system, may be characterized by a number of parameters, one of them being etendue. The etendue, ε, may be calculated using the following formula:ε=A*Ω≅π*A*sin2θ=π*A*NA2where Ω is the solid angle of emission or acceptance (in steradians); A is the area of the receiver or emitter, θ is the emission or acceptance angle, and NA is the numerical aperture. If the etendue of a certain element of an optical system is less than the etendue of an upstream optical element, the mismatch may result in loss of light, which reduces the efficiency of the optical system. Thus, performance of an optical system is usually limited by the element that has the smallest etendue. Techniques typically employed to decrease etendue degradation in an optical system include increasing the efficacy of the system (lm/w), decreasing the source size, decreasing the beam solid angle, and avoiding the introduction of additional aperture stops.
Traditional optics used in illumination systems have included various configurations, but their off-axis performance has been satisfactory only within narrowly tailored ranges. In addition, optics in traditional illumination systems have exhibited insufficient collection characteristics. In particular, if a significant portion of a light source's output emerges at angles that are far from the optical axis, which is the case for most LEDs, conventional illumination systems fail to capture a substantial portion of such light. In addition, although some traditional reflective collimators have acceptable collection characteristics, for example, elliptical and parabolic reflectors, such reflectors are usually characterized by rotationally symmetrical bias. Such a bias generally results in the rounding of the resultant image, as well as in lack of overall correspondence between a point on the light source and a point on the target plane, thus causing loss of order and degradation of etendue. These and other shortcomings prompted complicated designs of optical elements and systems, which involved, e.g., utilization of complicated aspheric surfaces and complex combinations of numerous elements.